Sound barrier

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U.S. Navy F/A-18 at transonic speed. The cloud is due to the Prandtl-Glauert singularity.

This is an article about the aviation term. For the 1952 film see The Sound Barrier (film). For the heavy metal band, see Sound Barrier.

In aerodynamics, the sound barrier is the transition at transonic speeds from subsonic to supersonic travel (usually refers to flight). The term came into use during World War II when a number of aircraft started to encounter the effects of compressibility, a grab-bag of unrelated aerodynamic effects, and fell out of use in the 1950s when aircraft started to routinely "break" the sound barrier.

1 History
2 Media
3 References
4 External links

[edit] History

The bullwhip, or stockwhip, was probably the first human-made object to move faster than sound. The tip of the whip breaks the sound barrier and causes a sharp crack—literally a sonic boom. Many forms of ammunition also achieve supersonic speeds.

[edit] Early problems

The tip of the propeller on many early aircraft may reach supersonic speeds, producing a noticeable buzz that differentiates such aircraft. This is particularly noticeable on the Stearman, and noticeable on the T-6 Texan when it enters a sharp-breaking turn. Note that this is in fact undesirable as the transonic air movement creates disruptive shock waves and turbulence. It is due to these effects that propellers are known to suffer from dramatically decreased performance as they approach the speed of sound. It is easy to demonstrate that the power needs to improve performance are so great that the weight of the required engine grows faster than the power output of the propeller. This problem was one of the issues that led to early research in jet engines, notably by Frank Whittle and Hans von Ohain, who were led to their research specifically in order to avoid these problems in high-speed flight.

Propeller aircraft were nevertheless able to approach the speed of sound in a dive. However this led to numerous crashes for a variety of reasons. These included the rapidly increasing forces on the various control surfaces, which led to the aircraft becoming difficult to control to the point where many suffered from powered flight into terrain when the pilot was unable to overcome the force on the control stick. The Mitsubishi Zero was particularly "well known" for this problem, and several attempts to fix it only made the problem worse. In the case of the Supermarine Spitfire, the wings suffered from low torsional stiffness, and when ailerons were moved the wing tended to flex in a such a way to counteract the control input, leading to a condition known as "roll reversal". This was solved in later models with changes to the wing. The P-38 Lightning suffered from a particularly dangerous interaction of the airflow between the wings and tail surfaces in the dive that made it difficult to "pull out", a problem that was later solved with the addition of a "dive flap" that upset the airflow under these circumstances. Flutter due to the formation of shock waves on curved surfaces was another major problem, which led most famously to the breakup of de Havilland Swallow and death of its pilot, Geoffrey de Havilland Jr.

All of these effects, although unrelated in most ways, led to the concept of a "sound barrier" that would make it difficult, perhaps impossible, for an aircraft to break the speed of sound.

There are, however, several claims that the sound barrier was broken during World War II. Hans Guido Mutke claimed to have broken the sound barrier on April 9, 1945 in a Messerschmitt Me 262. However, this claim is widely disputed by most experts as the Me 262's structure could not support high transonic, let alone supersonic flight and thus this claim lacks a plausible scientific foundation.^[1] Similar claims for the Spitfire and other propeller aircraft are even more suspect. It is now known that traditional airspeed gauges using a pitot tube give inaccurately high readings in the transonic, apparently due to shock waves interacting with the tube or the static source. This led to problems then known as "Mach jump".^[2]

[edit] Attempts to break the sound barrier

In 1942 the United Kingdom's Ministry of Aviation began a top secret project with Miles Aircraft to develop the world's first aircraft capable of breaking the sound barrier. The project resulted in the development of the prototype Miles M.52 jet aircraft, which was designed to reach 1,000 mph (417 m/s; 1,600 km/h) at 36,000 feet (11 km) in 1 minute 30 seconds.

The aircraft's design introduced many innovations which are still used on today's supersonic aircraft. The single most important development was the all-moving tailplane, giving extra control to counteract the Mach tuck which allowed control to be maintained at supersonic speeds. In the immediate post-war era new data from captured German records suggested that major savings in drag could be had through a variety of means such as swept wings, and Director of Scientific Research, Sir Ben Lockspeiser, decided to cancel the project in light of this new information. Later experimentation on the Miles M.52 design proved that the aircraft would indeed have broken the sound barrier, with an unpiloted 3/10 scale replica of the M.52 achieving Mach 1.5 in October 1948.

US efforts started soon after with the Bell XS-1. Also featuring the all-moving tail, the XS-1, later known as the X-1. In was in the X-1 that Chuck Yeager was the first person to break the sound barrier in level flight on October 14, 1947, flying at an altitude of 45,000 ft (13.7 km).

George Welch made a plausible but officially unverified claim to have broken the sound barrier on October 1, 1947 while flying an XP-86 Sabre. He also claimed to have repeated his supersonic flight on October 14, 1947, 30 minutes before Chuck Yeager broke the sound barrier in the Bell X-1 using the adjustable tail concept. Although evidence from witnesses and instruments strongly imply that Welch achieved supersonic speed, the flights were not properly monitored and cannot be officially recognized. (The XP-86 officially achieved supersonic speed on April 26, 1948.)

The sound barrier was first broken in a vehicle in a sustained way on land in 1948 by a rocket-powered test vehicle at Muroc Air Force Base (now Edwards AFB) in California. It was powered by 6000 pounds of thrust, reaching 1,019 mph.^[3]

Jackie Cochran was the first woman to break the sound barrier on May 18, 1953 in a Canadair Sabre, with Yeager as her wingman.

[edit] The Sound Barrier fades

As the science of high-speed flight became more widely understood, a number of changes led to the eventual disappearance of the "sound barrier". Among these were the introduction of swept wings, the area rule, and engines of ever increasing performance. By the 1950s many combat aircraft could routinely break the sound barrier in level flight, although they often suffered from control problems when doing so (Mach tuck). Modern aircraft can transition through the "barrier" without it even being noticeable.

By the late 1950s the issue was so well understood that many companies started investing in the development of supersonic airliners, or SST's, believing that to be the next "natural" step in airliner evolution. History has proven this not to be the case, but both the Concorde and Tupolev Tu-144 both entered service in the 1970s regardless.

Although the Concorde and Tu-144 were the first aircraft to carry commercial passengers at supersonic speeds, they were not the first or only commercial airliners to break the sound barrier. On August 21, 1961 a Douglas DC-8 broke the sound barrier at Mach 1.012 or 660 mph while in a controlled dive through 41,088 feet. The purpose of the flight was to collect data on a new leading-edge design for the wing.^[4] Boeing reports that the 747 broke the sound barrier during certification tests. A China Airlines 747 almost certainly broke the sound barrier in an unplanned descent from 41,000 ft to 9500 feet after an in-flight upset on February 19, 1985. It also reached over 5g. ^[5]

On October 15, 1997, in a vehicle designed and built by a team led by Richard Noble, driver Andy Green became the first person to break the sound barrier in a land vehicle. The vehicle called the ThrustSSC ("Super Sonic Car"), captured the record almost exactly 50 years after Yeager's flight.